The optical design benefits of diffractive optical elements (DOEs) include unique and efficient form factors and the ability to encode complex optical functions, such as optical power and diffusion into thin layers. Bragg gratings (also commonly termed volume phase gratings or holograms), which offer high diffraction efficiencies, have been used in devices such as Head Up Displays (HUDs) and Helm Mounted Displays (HMDs). An important class of Bragg gratings devices is known as a Switchable Bragg Gratings (SBG). An SBG is a diffractive device formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture, or holographic polymer-dispersed liquid crystals (HPDLC) mixture.
One attribute of HPDLC is that the phase separation produces high modulation of the refractive index in a layer of material, which in turn enables wide fields of view and high optical efficiencies. Liquid crystals have high refractive index differences along their different molecular cross sections; thus by aligning the molecules correctly with respect to the gratings planes, an efficient and broadband hologram may be attained. However, HPDLC material is susceptible to haze. HPDLC undergoes a “phase separation” during recording of the holograms in which the liquid crystal molecules get squeezed out of the polymer matrix to form microdroplets. While these microdroplets are desirable in that they provide the desired modulation of phase, if the microdroplets are not small enough or not uniform enough haze may easily form. Haze creates haloes around HUD symbology, adding to the overall “green glow” of the display.